Producers of paper, paper coatings, plastic compositions, and non aqueous surface coatings such as paints and film laminates use mineral materials such as calcium carbonate as property enhancing pigments or fillers in these products.
The appearance of paper, plastic compositions, and surface coatings, such as the whiteness and brightness of the product is significant, especially in the finer grades of printing and writing papers, plastics employed in consumer goods, and decorative surface coatings. The majority of these products require a calcium carbonate having the highest degree of whiteness and brightness.
Manufacturers of particulate calcium carbonates used in these applications supply calcium carbonate as a high solids slurry, e.g., a high solids ground calcium carbonate (GCC) slurry, which they bleach with FAS or sodium hydrosulfite, both FAS and sodium hydrosulfite, and the equivalents thereof, referred to hereafter as reductive bleaching agents. They have found, however, bleaching high solids GCC slurries with reductive bleaching agents raises the Brookfield viscosity, with the slurry thickening much faster than normal. The thickening of the slurry is severe enough to make it difficult, if not impossible to empty tank trucks or tank rail cars by gravity discharge when the bleached carbonate stands for more than about eight to about ten hours. This problem becomes more pronounced with more finely ground high solids GCC slurries bleached with reductive bleaching agents.
The present invention comprises the discovery that ions in solution resulting from the breakdown products of the reductive bleaching agents cause this thickening. These ions include ions of sulfur acids, sulfites, bisulfites, sulfates, and the like. Urea, another breakdown product of FAS bleaching, does not appear to cause this thickening since it does not ionize. These acidic anions also lower the pH of the high solids GCC slurries, which paper manufacturers try to avoid since this can adversely affect the properties (coating color) of the mixture of calcium carbonate and reductive bleaching agent used to coat paper.
It would therefore be an advantage to negate the effects of the sulfur acid ions such as sulfite, bisulfite, water soluble sulfate ions, and the like in the process, and make reductive bleaching a viable process for the treatment of high solids GCC slurries.